CrushWrapper.cc: remove some std::string::c_str() calls

Passing the result of c_str() to a function that takes
std::string as argument is slow and redundant.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
Merge pull request #145 from dalgaaf/wip-da-c_str

CrushWrapper.cc: remove some std::string::c_str() calls

Reviewed-by: Sage Weil <sage@inktank.com>